ChangeFeedProcessor: Refactors ShutdownAsync to abstract in base class

- Make ShutdownAsync abstract in DocumentServiceLeaseStoreManager
- Add no-op override in DocumentServiceLeaseStoreManagerCosmos
- Consolidate ShutdownAsync call to single line in ChangeFeedProcessorCore

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: kirankumarkolli

Microsoft.Azure.Cosmos/src/ChangeFeedProcessor/ChangeFeedProcessorCore.cs

@@ -80,15 +80,9 @@ public override async Task StopAsync()
             DefaultTrace.TraceInformation("Stopping processor...");
             await this.partitionManager.StopAsync().ConfigureAwait(false);
 
-            // ShutdownAsync persists in-memory lease state. Runs outside finally
-            // because: (a) partitionManager.StopAsync() rarely fails, and when it
-            // does the caller already receives that exception; (b) letting
-            // ShutdownAsync exceptions propagate ensures callers know if
-            // persistence failed — silently swallowing defeats the feature's
-            // guarantee.
-            await this.documentServiceLeaseStoreManager
-                .ShutdownAsync()
-                .ConfigureAwait(false);
+            // Persists in-memory lease state (no-op for Cosmos-backed leases).
+            // Runs after partitionManager.StopAsync() so exceptions propagate to the caller.
+            await this.documentServiceLeaseStoreManager.ShutdownAsync().ConfigureAwait(false);
 
             DefaultTrace.TraceInformation("Processor stopped.");
         }

Microsoft.Azure.Cosmos/src/ChangeFeedProcessor/LeaseManagement/DocumentServiceLeaseStoreManager.cs

@@ -36,9 +36,6 @@ internal abstract class DocumentServiceLeaseStoreManager
         /// Called when the processor is stopping. Allows implementations to perform
         /// cleanup or state persistence (e.g., exporting in-memory lease state).
         /// </summary>
-        public virtual Task ShutdownAsync()
-        {
-            return Task.CompletedTask;
-        }
+        public abstract Task ShutdownAsync();
     }
 }

Microsoft.Azure.Cosmos/src/ChangeFeedProcessor/LeaseManagement/DocumentServiceLeaseStoreManagerCosmos.cs

@@ -5,6 +5,7 @@
 namespace Microsoft.Azure.Cosmos.ChangeFeed.LeaseManagement
 {
     using System;
+    using System.Threading.Tasks;
     using Microsoft.Azure.Cosmos;
 
     /// <summary>
@@ -80,5 +81,10 @@ internal DocumentServiceLeaseStoreManagerCosmos(
         public override DocumentServiceLeaseCheckpointer LeaseCheckpointer => this.leaseCheckpointer;
 
         public override DocumentServiceLeaseContainer LeaseContainer => this.leaseContainer;
+
+        public override Task ShutdownAsync()
+        {
+            return Task.CompletedTask;
+        }
     }
 }